A piston ring used in a reciprocating internal combustion engine is, based on a function thereof, classified into a pressure ring and an oil ring. In an automobile engine or the like, the pressure rings are generally composed of a first pressure ring which is positioned closest to a combustion chamber side and a second pressure ring which is positioned at a side below the first pressure ring (crank chamber side).
The first pressure ring is positioned close to the combustion chamber and hence, the first pressure ring is subjected to high temperature and not sufficiently lubricated. Accordingly, the first pressure ring is required to have high strength and good sliding characteristics. Conventionally, a piston ring made of cast iron which has a hard chromium plating layer formed on a sliding surface thereof has been popularly used. Recently, however, along with the increase of demand for higher speed, higher output and lower fuel consumption with respect to an internal combustion engine, requirements for piston ring are getting severer year by year. Under such circumstances, with respect to the first pressure ring, the conventional cast-iron piston ring having the chromium plating layer is not sufficient in view of the strength and the wear resistance. Accordingly, in place of the cast-iron piston ring, a steel piston ring made of martensitic stainless steel with a nitriding has been widely used.
On the other hand, the sliding circumstance of the second pressure ring is not so severe compared with the first pressure ring and hence, a cast-iron piston ring made of such as flaky graphite cast iron has been used up to now and a shift to steel material has not been taken place. However, from a viewpoint of the prevention of the increase of the global greenhouse effect, an automobile engine is required to satisfy the further reduction of the fuel consumption so that the reduction of weight of the piston ring is desired.
In view of such circumstances, there has been observed a trend that a base material of the second pressure ring is shifted from cast iron to steel in the same manner as the first pressure ring thus producing a steel piston ring which is thinner and lighter than the cast-iron piston ring.
Here, while the first pressure ring and the second pressure ring are required to have a function of sealing a combustion gas, these pressure rings are also required to have a function of scraping a lubricant applied to an inner peripheral surface of a cylinder by an oil ring toward a crankcase side as an important function thereof. Accordingly, these pressure rings are required to satisfy a condition that the pressure rings easily ride on the applied lubricant during a piston upward stroke (when the piston moves upwardly toward a cylinder head) and easily scrape the lubricant during the piston downward stroke (when the piston moves downwards).
To satisfy such a requisite, it is desired that an outer peripheral sliding surface of the piston ring is formed into a barrel-face surface or has a diameter thereof decreased and inclined toward a combustion chamber side thus forming a so-called taper-face surface, and a lower-surface-side corner portion which is defined by a lower side surface and an outer peripheral sliding surface forms a so-called sharp edge having an a cute angle. Since the conventional cast-iron material exhibits the good machinability, it is easy to machine the piston ring in such a shape.
However, the machinability of the steel material is inferior to that of the cast iron material. Accordingly, it has been found that, in producing the above-mentioned piston ring having a sharp edge, when the base material of the first pressure ring and the second pressure ring is shifted from the cast iron to the steel, more burrs are frequently generated in grinding and lathe_turning of the piston ring and hence, the productivity is remarkably decreased.
In general, a steel wire material which constitutes a raw material of a steel piston ring having a rectangular cross section is manufactured by drawing and/or rolling a steel material as described in a publication entitled “Manufacturing method of piston ring material” and accorded “Japanese Patent Laid-open Publication No. 035173/1989”. The wire material is coiled and cut in a ring shape. Grinding, lathe turning and the like are done thereafter so as to manufacture piston rings having a given size.
Accordingly, as shown in FIG. 1 and FIG. 3, at a stage that only the coiling is done, each corner portion 2 of the piston ring 1 maintains a round corner with a radius of R which the wire material originally has. Accordingly, to manufacture the piston ring having a sharp edge, it is necessary to machined out the whole side surface or the whole outer peripheral surface of the piston ring piece by an amount of R by grinding or lathe turning.
However, as mentioned previously, since the steel material is tough, it is difficult to remove swarf or turnings from the piston ring thus increasing the generation of so-called “burrs”. Accordingly, compared with the use of the cast-iron material, to suppress the generation of the burrs, it is necessary to provide means such as reducing a grinding speed, making a depth of cut small and the like thus remarkably decreasing the machining productivity.
On the other hand, to enhance an oil scraping function of a piston ring, as disclosed in Japanese Patent Laid-open Publication No. 124204/2001, a piston ring, of which a groove is formed at a corner portion defined between an outer peripheral sliding surface and a lower side surface, but interrupted at abutting end portions, has been known. This groove is extending axially up to a barrel-face crest portion. However, a depth of the groove in the radial direction is equal to or more than 0.5 mm and hence, this constitutes one of usual piston rings having an interrupted undercut shape.
Although the publication does not specifically describe material of the piston ring, in view of a machining operation to manufacture the piston ring of the interrupted undercut shape, the piston ring may be made of cast iron. This can be supported by a fact that there is no specific suggestion with respect to a steel piston ring in the publication. When this known technique is applied to a steel piston ring, as mentioned previously, problems still remain with respect to the suppression of the generation of burrs and the machining productivity.
Further, to mention performances of the piston ring, it is necessary to provide additional shapes to the piston ring and hence, following problems arise.
(1) Since the undercut is large, it is necessary to interrupt the undercut at abutting end portions to cope with the oil consumption and a blowby gas from the abutting end portions.
(2) Since the piston ring is twisted in the use state when the undercut is large, it is necessary to form an inner cut in an inner peripheral side to establish a balance.
The present invention has been made in view of such drawbacks and it is an object of the present invention to provide a steel piston ring which exhibits superior oil scraping performance and good productivity.